In the summer of 1918 Margaret and Beatrice Jerome board the U.S.S. Mongolia, an ocean liner turned U.S. Navy troopship bound for wartime France. They intend to claim the remains of their brother--Margaret's fraternal twin--and accompany it back to the United States. Our family doesn't want Michael buried so far from home, Margaret tells fellow passenger Robert Butler.
Lieutenant Butler, a newly commissioned Medical Corps surgeon, continues to struggle with the mystery of parents who suddenly vanished soon after his fourteenth birthday. The experience makes him wary of close personal ties, but auburn-haired Margaret has captured his attention.
Unfortunately, his cautious advances cannot overcome the barriers surrounding her unspoken conflicts.
Michael couldn't be dead, she rationalizes; it just wasn't possible. If something terrible had happened to her twin brother, then she would have sensed his distress. Yet, nothing like that had occurred: there was no moment of realization, no sudden sense of doom. If Michael had suffered a violent death, then how could she have been so oblivious to such a tragedy? Had suppressed sibling envy deprived her of the psychic closeness presumably shared by twins? Haunted by guilt and devastated by the loss, Margaret stifles her nascent feelings toward Butler.
At Le Havre, they bid each other reluctant farewells.
In Paris, the sisters discover surprising particulars about their brother's life and his work as an ambulance driver with the American Field Service. Complicating their task is a recent codicil to Michael's Last Will and Testament: a legally sound document that frustrates their attempt to claim his remains. More remarkable discoveries follow, including his intimate relationship with a nurse and a disturbing revelation surrounding the unsolved murder of a French artillery officer.
Michael had not shared those events with his twin sister, nor with anyone in his family. Astonished, hurt, and confused, Margaret comes to realize that no matter how close two people might seem to be, one cannot truly know the mind of another human being.
Meanwhile, the reality of war challenges Butler's sense of who he is. Saddled with the battlefield memory of his shameful behavior toward a horribly mangled soldier, Butler resolves never to reveal what happened. Every human being has a terrible secret they cannot share, he tells himself. This will be mine.
Despite her initial coolness aboard ship, Butler writes to Margaret, hoping for a positive response. To his surprise and delight she agrees to meet him in Paris should he get leave from his duties at the front. It is the turning point in both their lives.
The Armistice finds them together on the French Riviera, amidst the wealthy and the wounded, deeply in love but harboring memories and secrets that leave them forever changed.